1. Field of the Invention
This invention relates generally to structural fasteners, and more particularly relates to drive sockets on threaded male structural fasteners.
2. Description of the Related Art
Threaded male structural fasteners of the type to which this invention is directed include an axial drive socket, or wrench engaging recess, located either on a head (FIG. 1a) of the fastener or on the opposite end of the fastener (FIG. 1b). The drive socket (FIG. 1c) includes a plurality of substantially equally spaced and sized flat surfaces which receive a mating wrench type key, or wrench key. The drive socket may be hexagonally shaped to receive a hexagonal Allen wrench type key, or hex-key driver. During use, an end of the wrench key is inserted into the drive socket in the male fastener and is either held in place to permit a threaded female fastener, such as a nut, to be screwed onto the threads of the male fastener, or the hex-key driver is rotated in order to screw the male fastener into the female fastener or other female threaded hole.
Clearance is required between a hex-key driver and the drive socket which the hex-key driver is inserted into. For example, the clearance between a 2.4 mm ({fraction (3/32)}-inch) hexagonal drive socket and a 2.4 mm ({fraction (3/32)}-inch) hex-key driver is 0.04 mm (0.0015 inch) all around. With this amount of clearance, the hex-key driver can rotate approximately three degrees before contacting the mating surfaces of the drive socket (FIG. 1d), resulting in the intersections, or corners, between the surfaces of the hex-key driver contacting the flat surfaces of the drive socket. With the corners between the surfaces of the hex-key driver providing the main driving force from the hex-key driver, high stress concentrations are placed on the corners of the hex-key driver, but there is very little material providing support for the corners. As a result, when a load that is sufficient to set the desired torque between the male threaded fastener and the female threaded fastener or part is applied to a hex-key driver during use, the material at the corners may shear off of the hex-key driver, leaving the hex-key driver in a stripped condition.
Another problem that can occur is that as the hex-key driver is rotated within the drive socket, the hex-key driver may become wedged within the socket, thus causing the hex-key driver to bind within the socket. After setting the desired torque between the male threaded fastener and the female threaded fastener, rotation of the hex-key driver must be reversed to break the bind between the hex-key driver and the socket.
Methods which have been developed to prevent the stripping of hex-key drivers during use include fabricating the hex-key drivers from stronger materials. The present invention reduces the need for a special high-strength hex-key. Another method of preventing hex-key drivers from stripping includes making the drive sockets in the fasteners deeper. However, making the drive socket deeper may compromise the tensile strength of the fastener through the area of the drive socket, especially on fasteners having the drive socket on the end of the fastener opposite the head (FIG. 1b).
One attempt to alleviate the problem of the hex-key drivers stripping within the drive sockets included reconfiguring the drive socket to include six rounded lobes of substantially equal radius, as depicted in FIG. 2a. The lobes are located substantially equidistant from the center of the fastener. Also, adjacent lobes are located substantially equidistant circumferentially from each other. Each of the lobes projects inward toward the center of the fastener. The embodiment also includes six curved recesses located between and adjacent each lobe, thereby combining for a total of six recesses. Each of the recesses is blended into its adjacent lobes to form a substantially smooth transition between the lobes and the recesses.
Referring to FIG. 2b, when the hex-key driver is rotated, such as to drive the male threaded fastener relative to a female threaded fastener, a portion of the flats on the hex-key driver contact the lobes while the corners of the hex-key driver remain within the recesses and out of contact with the wall of the drive socket. One problem with the configuration of FIGS. 2a and 2b is that it enlarges the drive socket considerably, thereby weakening the fastener head.
Hence, those skilled in the art have recognized a need for a male threaded fastener having a drive socket that may receive high stress from a hex-key driver and reduce the tendency of the hex-key driver to strip or to bind within the drive socket. The need to configure the drive socket so that it substantially maintains the tensile strength of male threaded fasteners and the strength of the fastener head has also been recognized. The present invention fulfills these needs and others.
Briefly, and in general terms, the present invention is directed to a drive socket for a fastener which is driven by a hex-key driver. In one presently preferred embodiment, the drive socket includes three rounded lobes of substantially equal radius. The three lobes are positioned about a center of the fastener and are located substantially equidistant from the center and approximately 120xc2x0 apart from each other. The drive socket also includes three flat surfaces positioned about the center of the fastener. Each flat surface is located substantially opposite one of the lobes and substantially equidistant from the center of the fastener. The drive socket further includes recesses which are located between the lobes and the flat surfaces.
In one currently preferred aspect of the invention, the lobes project inward toward the center of the fastener. Each of the recesses forms a substantially smooth transition between a lobe and a flat surface. The lobes are positioned to provide a first clearance between an apex of each of the lobes and corresponding surfaces on a hex-key driver inserted into the drive socket. The flat surfaces are positioned to provide a second clearance between the flat surfaces and corresponding surfaces on the hex-key driver. In one currently preferred embodiment, the first clearance is approximately 0.04 mm (0.0015 inch) and the second clearance is approximately 0.10 mm (0.0038 inch). In another currently preferred embodiment, the recesses are sized to provide sufficient clearance such that the corners of a hex-key driver inserted into the drive socket do not contact a wall of the drive socket.
In another presently preferred embodiment, the drive socket of the present invention includes a first set of three lobes of substantially equal radius which are positioned about a center of the fastener. The lobes within the first set are substantially equidistant from the center of the fastener and approximately 120xc2x0 apart from each other. Each of the lobes of the first set includes a flat surface which is located adjacent and substantially tangential to the radius portion of the lobe. The drive socket also includes a second set of three lobes of substantially equal radius positioned about the center of the fastener. The lobes within the second set are substantially equidistant from the center of the fastener, approximately 120xc2x0 apart from each other and approximately 60xc2x0 apart from the lobes of the first set. The drive socket further includes recesses located between the lobes of the first set and the lobes of the second set.
In one currently preferred aspect of the invention, each of the lobes of the first set and each of the lobes of the second set projects inward toward the center of the fastener. In another currently preferred aspect of the invention, the radius of the lobes of the first set and the radius of the lobes of the second set are substantially equal. The distance between the lobes of the first set and the center of the fastener and the distance between the lobes of the second set and the center of the fastener are also substantially equal. Each of the recesses forms a substantially smooth transition between the lobes of the first set and the lobes of the second set. The recesses are sized to provide sufficient clearance such that the corners of a hex-key driver inserted into the drive socket do not contact a wall of the drive socket. The flat surfaces of the lobes in the first set are positioned substantially 120xc2x0 apart from each other.
In another currently preferred aspect of the invention, the lobes of the first set are positioned to provide a first clearance between an apex of each of the lobes of the first set and corresponding surfaces on a hex-key driver inserted into the drive socket. Similarly, the lobes of the second set are positioned to provide a second clearance between an apex of each of the lobes of the second set and corresponding surfaces on the hex-key driver inserted into the drive socket. The first clearance and the second clearance are both approximately 0.04 mm (0.0015 inch).
From the above, it may be seen that the present invention provides for configurations of drive sockets for fasteners which reduce the tendency of the hex-key driver to strip or to bind within the drive socket. Further, it may also be seen that the drive socket substantially maintains the tensile strength of the fastener and the strength of the fastener head. These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.